Two-pole DC voltage sources with a variable DC voltage on the output side are known, for example, from fuel cell applications, in which the fuel cell represents such a DC voltage source with a variable DC voltage. Normally, such a DC voltage source supplies a three-point DC voltage intermediate circuit owing to the different DC voltage level, which three-point DC voltage intermediate circuit is formed by a first capacitor and a second capacitor connected in series with it. One connection of the first capacitor also forms an upper connection of the three-point DC voltage intermediate circuit, and one connection of the second capacitor forms a lower connection of the three-point DC voltage intermediate circuit. A center point connection of the three-point DC voltage intermediate circuit is also formed at the point at which the first capacitor is connected to the second capacitor. A converter circuit is normally connected to the upper connection, to the center connection and to the lower connection of the three-point DC voltage intermediate circuit, and the electronic switches in this converter circuit are driven by a drive circuit.
A converter circuit such as this having a DC voltage source with a variable DC voltage and a three-point DC voltage intermediate circuit connected to the converter circuit is specified in EP 01810944.7, which has not yet been published. A converter is provided for coupling to an electrical DC voltage supply network, and is connected on the DC voltage side to the three-point DC voltage intermediate circuit and on the AC voltage side to the electrical AC voltage supply network, in particular via a transformer. If a first intermediate circuit voltage across the first capacitor is not equal to a second intermediate circuit voltage across the second capacitor, the converter circuit is driven such that both the first and the second intermediate circuit voltages are set to an intermediate circuit voltage nominal value by regulation. First of all, for this purpose, an intermediate circuit voltage mean value is formed from the first intermediate circuit voltage and from the second intermediate circuit voltage, and this mean value is then regulated at the intermediate circuit voltage nominal value. However, this regulation process is dependent on the converter circuit being able to draw electrical power from the DC voltage source, that is to say the DC voltage source must continuously supply electrical power for the regulation process and, accordingly, is required to compensate for the abovementioned inequality. If the DC voltage source fails, such compensation is, however, no longer ensured. If, furthermore, the converter is used as a pure power factor corrector for the electrical AC voltage supply network and an inequality occurs in the three-point DC voltage intermediate circuit as described above, then this imbalance can be compensated for only in the manner described above, provided the converter circuit can draw electrical power from the DC voltage source, that is to say the DC voltage source continuously supplies electrical power for the regulation process.